1. Field of the Invention
The invention concerns a method of handing off traffic in a low orbit satellite communication system serving terminals which may or may not be mobile and which are transmitter-receivers or receivers only, a terminal being connected to a terrestrial communication network by means of a connection station.
The invention also concerns a communication system using a method of this kind.
2. Description of the Prior Art
Satellite-based communication with mobile terminals has until now used two types of orbit: geostationary satellite orbits and highly inclined elliptical orbits, both kinds being situated, on average, above areas known as the "Van Allen belts" characterized by a high concentration of particles. More recently lower orbits have been envisaged. Their altitude is between 800 and 2000 km. One feature of satellite communication systems using these orbits is the possibility of communicating with a large number of mobile terminals, portable terminals, for example. The difference between orbits at altitudes above the "Van Allen belt" and those at lower altitudes is that the closer to the earth the satellite is located the lower is the attenuation.
A CCIR report (document US IWP 8/14-52, Aug. 1, 1990) entitled "Technical characteristics of a personal communication mobile satellite system" describes a low orbit satellite communication system using multibeam antennas. The resulting constellation of satellites comprises 77 satellites to provide global coverage. It is made up of seven planes each of eleven satellites each in a circular polar orbit. However, to maintain a call when a terminal leaves the coverage area of a first satellite, a system of this kind includes intersatellite links. A solution of this kind requires demodulation on board the first satellite, routing of the information, transmission to a second satellite and relay to the connection station. This is very complex and costly.
An object of the present invention is to minimize the cost of the space segment by using transparent satellites and eliminating all intersatellite links.